Pacemaker methods and pacing control systems operable from a preferred one of at least two pacing rate control signals

ABSTRACT

This pacing system relates to a rate varying cardiac pacemaker (1) for electrically stimulating the heart of a pacemaker wearer. The electrical cardiogenic heart depolarization is detected on a multipolar single probe with the aid of individual electrodes (7 to 10). Bipolar electrodes in the atrium allows for detection of the intra-atrial actions (P-wave) as a first rate control signal for controlling the pacemaker in the atrially triggered ventricular pacing mode (VDD). A second rate control signal other than the P-wave correlating with patient activity is determined in parallel with the P-wave control signal. The two control signals are compared to decide if the intra atrial actions (P-waves) are appropriate control signals. Pace rate control is switched between the two signals in order to produce the most beneficial pacing mode to the patient, for example, VDD or VVI rate responsive modes. This control system can respond to unreliability or instabilities of the intrinsic atrial P-wave signal, for example, fibrillation and shift control to another appropriate rate responsive mode.

TECHNICAL FIELD

The present invention relates to a cardiac pacemaker having a pulsegenerator for generating pacing pulses at a certain pacing rate, atleast one pacing electrode disposed in the heart for receiving thepacing pulses, at least two measuring electrodes also disposed in theheart for detecting electrical parameters influenced by physiologicalquantities due to the patient's exercise, an evaluation circuit for themeasuring electrode signals for determining a control signal adapted tothe patient's exercise, and a control circuit for varying the pacingrate in accordance with the patient's exercise.

BACKGROUND ART

In the early years of pacemaker therapy, simple systems stimulating inthe ventricle were used. This was due to the technical possibilitiesexisting in the early sixties. Nevertheless, one soon recognized thevalue of an atrioventricular synchronization from a hemodynamic point ofview. As early as 1963, Nathan described the application of an atriallytriggered ventricle pacing system in the article "An implantablesynchronized pacemaker for the long-term correction of complete heartblock"0 in Circulation 23, 1963, pp. 682ff. This article made clear theadvantages of rate control with simultaneous atrioventricularsynchronization by detecting the atrial potential. Such pacemakers areadvantageous in particular for patients with a complete AV block, i.e.if the stimulus conduction system in the heart is interrupted, thatmaintains the normal rhythm if the sinus node function still exists atthe same time. Problems in reliably detecting the atrial signal led inpast years to the development of an atrial screw electrode which is"screwed" into the atrial wall with a corkscrew-like spiral or the like.As of the mid seventies it became possible to detect the atrial signalswith so-called VAT pacemakers and use them to trigger cardiacstimulation in clinical application.

However, the theoretical advantage of applying such atrially triggeredVAT systems, which are known today as DDD systems or dual chambersystems, is opposed by considerable problems in practice. Firstly, theproblem of firmly anchoring the electrodes in the atrium has not beensatisfactorily solved, whether by screw electrodes or by inserting anelectrode into the auricular appendix. The second problem relates to theinstability of the atrial rhythm. A survey of this can be found in thebook by E. Alt, "Schrittmachertherapie des Herzens," perimed Verlag,Erlangen (1988), pp 94 to 99 . In many patients showing a disturbance ofstimulus conductance, there are also disturbances in the stimulusformation. This means that, alongside an occasionally regular sinusrhythm, there may be fast atrial arrhythmias in the sense of atrialfibrillation or an atrial flutter, but that sinus node dysfunction mayalso express itself as an excessively slow sinus node function. This isreferred to as a "brady-tachy syndrome" or , if the AV conduction isadditionally disturbed, as binodal disease. In the case of fast atrialarrhythmias the ventricle is paced inadequately in the rhythm of theatrial arrhythmia; in the case of sinus node damage and a deficient risein the sinus rate the pacing rate for the ventricle is too slow.Technical restrictions with respect to a reliable detection of theatrial signal and due to the limited expressive power of the atrialsignal lead, for a considerable number of patients, to restrictions inthe dual chamber or DDD systems used at present.

To avoid the problems with respect to an unstable anchoring of theatrial electrode, it has been proposed to provide a floating pair ofelectrodes in the atrium within one pacing lead. This concept wasalready presented in 1979 by Antonioli in the book "Cardiac pacing, PACESymposium, Montreal," edited by C. Meere, in the article "A simpleP-sensing ventricle stimulating lead driving a VAT generator." U.S. Pat.No. 4,313,442 (Knudson) describes a corresponding pacemaker whichintegrates the atrial pulse signals and adjusts a pacing rate inaccordance with the integrated atrial signal. This method does not allowfor direct synchronization between the atrial beat and the ventricularbeat, but for an adaptation of the ventricular beat rate to the meanatrial rate.

The concept of detecting the atrial signals indirectly with a multipolarelectrode corresponds to the technique, that has been practiced for manyyears in electrophysiology with transitory electrodes, of detecting theatrial signal with bipolar, quadripolar or six-pole floating electrodesthat pass through the atrium by lying in the blood stream and are notsecured by direct contact to the atrial wall. Two electrode points, i.e.a pair of electrodes in the ventricle and one or two pairs of electrodesin the atrium, are generally used within a common electrode body. Morerecent developments in electrode technology also allow for theapplication for thin multipolar electrodes whose diameter is only about1.6 millimeters.

In the past it was not easy for the evaluating electronics of cardiacpacemakers, however, to detect these indirectly detected signals of theatrium with a floating electrode position. But now that microprocessortechnology has made progress, it is possible to derive a control signalfor ventricular pacing triggered in synchronism with the atrium within aDDD system by means of suitable input filters and input amplifiers aswell as appropriate processing of the signal perceived indirectly in theatrium from floating electrode points within the blood stream.

When the problem of detecting the atrial signal with one electrode notin direct contact with the atrial wall is solved, however, one still hasthe problem of the instability of the cardiogenic atrial signal, which,as explained above, may be both too fast and too slow. Since theventricular rate is adapted to this atrial signal in a classic DDDpacemaker, this may result in an inadequately fast or slow ventricularrate.

To obtain, independently of the atrial rate, a correct pacing rate inthe ventricle that is adapted to the patient's exercise, variousconcepts for rate adaptive pacemakers have been proposed in the past.

Krasner describes in U.S. Pat. No. 3,593,718 a pacemaker in which theexternal thoracic impedance is measured, and the breathing rate detectedtherefrom and used for controlling the pacing rate.

Nappholz describes in U.S. Pat. No. 4,702,253 a pacemaker which providesa measuring current for measuring the impedance in order to use thisimpedance measurement to detect the breathing and use it for controllingthe pacing rate.

Salo applies, according to U.S. Pat. No. 4,686,987, a similar impedancemeasuring method to determine the stroke volume of the right ventricle.

Lekholm also uses an impedance measurement for detecting the breathingrate according to U.S. Pat. No. 4,697,591.

In U.S. Pat. No. 4,694,830, the same inventor describes a pacemaker inwhich the particular impedance can be detected from the change in thestimulation voltage and the stimulation current during each generatedstimulation pulse by division of the two stated values, and from thechange therein the breathing rate can be indirectly detected, the latterthen being used to control the pacing rate. However, this is onlysuccessful in cases in which pacing is effected only by the pacemaker.Cardiogenic heart beats cannot be used with this method for rate controland for detecting the breathing rate. This is an essential limitation ofthis system.

H. Strandberg, et al., in U.S. Pat. No. 4,757,815 controls pacing inaccordance with amplitudes of QRS complex signals and varies the rate ofthe pacing pulses in response to shifting frequency of the respirationsignal as acquired from fluctuations in the heart signal derived betweena single electrode tip in the heart and a pacemaker housing. Respirationsignals are thus subject to false variations such as by swinging thearms. Furthermore there is no fail-safe check of the respiration rateagainst any other sensed values to determine if it is an optimal controlsignal for pacing when starting or stopping strenuous exercise forexample when the breathing rate is not necessarily an optimal indicatorfor pacing rate.

Most of the aforesaid systems use the detectable change in impedance todetermine the breathing rate and use it for control of the pacing rateof the heart. An atrially synchronous control of the heart rate in theventricle is not possible with such systems alone, since they have nosuitable measures for detecting the atrial activity as well.

The invention is based on the problem of reliably controlling a cardiacpacemaker of the type in question with control signals for the pacingrate obtained from a selected optimum one of different cardiacactivities with simple measuring technology and low operative effort andenergy resources, and which furthermore provides the possiblity ofperceiving the signals of the atrium without an additional atrialelectrode and using only one probe to the heart.

DISCLOSURE OF THE INVENTION

This problem is solved according to the invention by determiningamplitude variations of the intracardiac cardiogenic signal due to thepatient's breathing which are determined by the orientation of thevector of the cardiogenic stimulus current with respect to the axis ofperception defined by the position of the measuring electrodes, andderiving therefrom the pacing control signal.

The invention thus makes use of the fact that the resultant of allcardiogenic electrical currents during depolarization of the myocardialcells, i.e. the so-called sum vector, has a determinating quantity anddirection during the excitation process. If this vectorial electricstimulus, i.e. the intracardiac signal passing through the heart, isdetected in terms of voltage and amplitude with the aid of a bipolarelectrode, differential delays occur between the individual signalsdepending on the orientation of the bipolar electrode with respect tothe vectorial electric field. If the two electrode points of the bipolarelectrode are parallel to the vector, a bipolar signal is generatedwhich exceeds the unipolar signal in terms of amplitude. If the bipolarelectrode is disposed at right angles to the direction of conduction ofthe electrical stimulus, virtually isochronic potentials occur on thetwo individual electrodes, which subtract from each other. Since theorientation of the vectorial electric field during the excitationprocess is determined by the instantaneous geometry of the heart, whichis in turn a function of the breathing and thus of the position of theheart within the thorax in accordance with the diaphragmatic state, andthe resulting change in the relation between the sum vector and thebipolar electrode position, one can determine the breathing byevaluating the amplitude variations of the bipolar intracardiac signal,and use it as a regulating variable for adjusting the pacing rate. Thebreathing, preferably the breathing rate, but also the breathing rateand breathing depth, can be used for this adjustment either alone or inconjunction with a further control parameter, whereby it is within thescope of the invention to use the breathing primarily or onlysupportively for this control. In the latter case the further controlparameter is used primarily for adjusting the pacing rate. This takesaccount of the fact that cardiac pacemakers having several controlparameters either detected by sensors or by detection of the naturalatrial P wave are being increasingly used today because this can clearlyincrease the reliability of an adequate pacing rate.

The detection of the intracardiac signal can take place in theventricle, the atrium or between the atrium and the ventricle. Theamplitude variations are smallest upon detection in the ventricle, andgreatest upon detection between the ventricle and the atrium. If onlythe electrical atrial signal is detected, an atrially triggered, aso-called VDD pacemaker with pacing of the ventricle can be used forpacing under control of the atrial P wave. The evaluation circuit of thepacemaker can then perform a comparison between the pacing ratecalculated by the atrially triggered VDD method and the pacing ratecalculated on the basis of the vectorial amplitude variation of thestimulus field and correlating with the breathing rate as acomplimentary sensor control signal. After this comparison theevaluation circuit selects the suitable pacing rate for pacing theventricle. An atrioventricular synchronization in accordance with theperception of atrial actions is also possible in the case of respirationtriggered rate adaptive pacing.

In a pacemaker at least two electrode points are preferably disposed inthe atrium and at least one electrode point in the ventricle. Theevaluation circuit can then compare the perceived vectorial amplitudevariations between various electrode points, thereby allowing fordetection of the most favorably situated electrode points fordetermining the breathing individually.

In order to avoid interference influences from pacing pulses duringdetection of the electrical intracardiac signal, one picks up theintracardiac signal only a certain time after a pacing pulse has beenprovided ("blanking"). One can also determine the vectorial amplitudevariations of the intracardiac signal by performing a signal correctionfor paced and perceived cardiogenic actions.

Upon detection of atrial instability such as atrial fibrillation one canalso perform a strictly rate adaptive ventricular pacing (VVI) with rateadaption in accordance with the vectorial amplitude variation of theintracardiac signal. The evaluation circuit can additionally perceiveinadequate atrial tachyarrhythmias which are then not used forinfluencing the pacing rate. This would be the case for example if ahigh atrial rate is detected but the sensor signal indicates no state ofphysical exercise. In such a case the pacemaker automatically switchesover to a new mode of operation in which rate adaptive ventricle pacingis performed in accordance with the vectorial amplitude variation of theintracardiac signal or in accordance with a further rate adaptivecontrol parameter such as body activity detected by a motion sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of invention can be found throughout thefollowing description, drawings and claims.

The invention shall be explained in more detail in an exemplaryembodiment with reference to the drawing, in which

FIG. 1 shows a schematic view of an inventive cardiac pacemaker havingan implantable pacemaker can and a probe connected to four individualelectrodes and directed via the atrium into the right ventricle;

FIG. 2 shows a schematic block diagram to explain the basic function ofthe inventive pacemaker;

FIG. 3 shows a schematic view of the position of individual electrodesin the heart with respect to the wave front of the electricalintracardiac signal;

FIGS. 4a, 4b, 4c show signal diagrams, picked up directly on a patient,of the electrical intracardiac signal, the breathing rate derivedtherefrom and the directly measured breathing of the patient;

FIG. 5 shows a more detailed block diagram of an inventive pacemaker toexplain further functions; and

FIG. 6 is a block diagram of a typical preferred signal logic evaluationsystem, as provided by this invention.

THE PREFERRED EMBODIMENTS

As seen from FIG. 1 pacemaker system 1 comprises an implantable can 2containing evaluation and control electronics 3 shown in FIG. 2, and aprobe 4 connected to the pacemaker and introduced via right atrium 5into ventricle 6 of a human heart. The probe has four electrodes 7, 8, 9and 10, electrode 7 being situated at the tip of the probe, electrode 8in the ventricle and electrodes 9 and 10 in atrium 5. Probe 4 isconnected to the pacemaker can via a quadripolar connection 11 withterminals 12, 13, 14 and 15 indicated here only schematically.

The ventricle is paced e.g. in bipolar fashion via the two singleelectrodes 7 and 8 situated in the ventricle. The cardiogenic electricalsignal during excitation is detected with the aid of two singleelectrodes, e.g. between electrodes 7 and 8 in the ventricle, betweenelectrodes 9 and 10 in the atrium or perhaps between single electrode 8in the ventricle and one of electrodes 9 or 10 in the atrium. Thisdetection is strictly passive, requiring no energy supplied from theoutside. The two single electrodes detect an electrical intracardiacsignal which is indicated schematically as 20 in FIG. 2. Thisintracardiac signal is fed to a frequency filter 21 and thereafter to asignal shaper 22. The frequency filter has a frequency range adapted tothe actually occurring breathing rates, and detects frequencies between0.1 Hz and about 1 Hz with a peak at about 0.35 Hz, thus having a lowpass function. Endogenic and exogenic signals are thereby extracted,e.g. the patient's heart rate or physical jolts. A preceding high passfilter of 0.1 Hz serves to stabilize the baseline. The intracardiacsignal corresponds to an ECG signal with the known P- to T-wavecomponents. Due to the low pass filtering, a signal 23 correlating withthe breathing is directly applied to the output of the signal shaper;cf. also the second lines in FIGS. 4a to 4c. A control signal is derivedfrom this signal in the known manner in an evaluation logic 24, saidcontrol signal being fed via a line 25 to a control circuit 26 for apulse generator 27. Pulse generator 27 then provides pacing pulses 28 ata pacing rate adapted to the patient's particular exercise determined bythe breathing. In the case of unipolar pacing of ventricle 6, pacingpulses 28 are provided to electrode 7 and to the pacemaker can as anopposite electrode; in the case of bipolar pacing they are provided toelectrodes 7 and 8 in the ventricle.

FIG. 3 shows a schematic view of the electrical processes within theheart in the case of bipolar perception, with two different positions ofprobe 4 within the atrium. These two positions are referred to as 4-1and 4-2. The two electrodes are referred to schematically as 9 and 10.The electrical wave front following excitation of the heart can berepresented as an arrangement of dipoles 30 which spread in accordancewith arrow 31 in the direction of depolarization. In position 4-1 of theprobe, i.e. when it is oriented parallel to the direction ofdepolarization, a positive potential first occurs at electrode 9 and isfollowed by a negative potential. Due to a time delay in the occurrenceof this signal at electrode 10, caused by the transit time, a bipolarsignal is generated which exceeds in terms of amplitude the particularsignals detected in unipolar fashion; cf. the upper left signalrepresentation in the figure. The bipolar perception acts in the mannerof a differential amplification. If the probe is in position 4-2, i.e.at right angles to the direction of conduction of the stimulus front,isochronic potentials occur at electrodes 9 and 10 which subtract eachother; cf. the upper right signal diagram in FIG. 3.

During operation of the pacemaker the position of probe 4 can be assumedto be relatively stationary, while the vector of the electrical wavefront following excitation represented by arrow 31 changes with time,this change in time correlating with the breathing. Obviously, anevaluation of the amplitude variations of the signal detected in bipolarfashion results directly in a signal corresponding to the breathing.

FIGS. 4a, 4b and 4c show original recording signals taken from patients.

FIG. 4a shows in the uppermost line intracardiac signal 20 which isdetected in bipolar fashion in the atrium with an intrinsic sinus rhythmat a slow rate with the aid of single electrodes 9 and 10. Low passfiltering and (optional) signal shaping yield signal 23 determined inthe second line, that is directly correlated with the breathing rate.The lowermost line shows a signal 40 corresponding to the directlymeasured breathing signal. One can see that signals 23 and 40 correlatevery well with each other; there is a small apparent delay in thefiltered signal relative to the directly measured breathing, which isdue to the low pass filter.

FIG. 4b shows in the first line intracardiac signal 20 which was alsodetected with the aid of electrodes 9 and 10 in the atrium with pacingin the ventricle at 70 pacing pulses per minute. Signal 23 derivedtherefrom again correlates well with directly measured breathing signal40.

In FIG. 4c intracardiac signal 20 was detected between the ventricle andthe atrium. Signal 23 derived therefrom also correlates well with thedirectly measured breathing according to signal 40.

In order to select the particular most favorable signal 23, one canprovide before frequency filter 21 (FIG. 2) a selection circuit 41 whichis triggered by evaluation logic 24 and connects two selected electrodeswith frequency filter 21. One can also integrate into this selectioncircuit 41 an extraction circuit which connects the selected electrodeswith frequency filter 21 only a short time after a pacing pulse has beenprovided, thereby avoiding interference influences on the measurement bythe relatively high energy of the pacing pulses.

FIG. 5 shows a block diagram circuit for a portion of an atriallytriggered pacemaker. Atrial electrodes 9 and 10 are connected to a Pwave detector 51 which detects the P wave of the intracardiac signal.The output signal thereof is fed to a delay circuit 52 which provides asignal to a pulse generator 53 after about 150 milliseconds. This pulsegenerator provides a first pulsed control signal 54 to a comparator andselection circuit 55.

In a second branch of evaluation circuit 3' a rate adaptive controlparameter is calculated by the above-described method in an evaluationcircuit 56 from the signals of further electrodes, e.g. ventricularelectrodes 7 and 8 or a ventricular electrode and an atrial electrode,and fed to a pulse generator 57. The latter provides a pulsed secondcontrol signal 58 to comparator and selection circuit 55, which in thiscase corresponds to the above-described control signal correlating withthe breathing. It is also possible to evaluate the signals of anothersensor in this second branch of the evaluation circuit and to calculatetherefrom a rate adaptive control parameter which does not necessarilycorrelate with the breathing. Such control parameters may be determinede.g. from the pacemaker wearer's activity, the oxygen saturation or thetemperature of the venous blood, etc.

Comparator and evaluation circuit 55 decides which of the controlsignals 54, 58 is passed on to control circuit 26 for the pulsegenerator, which then provides corresponding pacing signals 28 to theventricular electrodes. In the atrially triggered pacemaker mode, firstcontrol signal 54 is usually passed on by comparator and evaluationcircuit 55. Only when this signal shows no clear values or fails tocorrelate with an expected value according to the information of thesecond control signals 58 this is then used to control the pacing rate.Those conditions exist for example if the atrial rate is too slow(following newly developed virus mode disease) or too fast followingatrial dyrythmias such as atrial flutter or atrial fibrillation. In thiscase the second control signal 58 gains exclusive control over thepacing rate.

The ventricular signals provided by ventricular electrodes 7 and 8 areevaluated in a third branch of evaluation circuit 3'. The electricalsignal is fed to a QRS detector 59 which detects the QRS wave followingthe P wave in the intracardiac signal. The detector signal is fed to afirst input of an inhibition circuit 60 whose second input is connectedto the output of P wave detector 51. Inhibition circuit 60 additionallyreceives from delay circuit 52 a pulse which characterizes the timedelay of about 150 milliseconds preset there. Inhibition circuit 60 thenprovides a pulsed inhibition signal 61 to comparator and evaluationcircuit 55 and blocks it when the output signal of QRS detector 59 iswithin the time span of 150 milliseconds preset by delay circuit 52.This therefore occurs only upon a cardiogenic beat, so that pacing isunnecessary in this case. The QRS wave usually follows the P wave afterabout 130 milliseconds in the case of cardiogenic rhythm.

The comparison and selection logic of block 55 is explained in moredetail in connection with the logic diagram of FIG. 6. Thus thepreferred pulse 54 dominates and goes to the pulse generator controlcircuit 26 provided sensor 71 shows that the signal is normal andproper. If not the lead 80 actuates NOT circuit 72 and blocks the signal54 from OR circuit 73. Also the signal at 80 operates to pass throughAND circuit 74 the alternative signal 58, which proceeds through ORcircuit 73 in place of signal 54. The sensor 71 for example could detectthe presence of dysrythmia such as fibrillation or flutter.

Thus signal pulse 75 is the survivor of signals 54 and 58 which proceedsto the output control circuit 26 via NOT circuit 77 and OR circuit 79 ifpulse analysis sensor 76 indicates that the pulse train is good and thusdoes not inhibit the signal at NOT circuit 77 over lead 81. A furtheralternative pulse 70 can be selected if neither pulses 54 or 58 areavailable via AND circuit 78 in the presence of a signal on lead 81,when pulse 75 is unsatisfactory. Thus, the final pulse 85 is thesurvivor of pulses 75 and 70 as processed by OR circuit 79.

Thus, this invention provides a pacemaker evaluation system that mayselect a preferred one of several signal choices available at the seriesof electrodes in the heart or other sensing means for pacing. This forexample provides for selection of the best available atrial P-wavesignal 54 or vector signal 58 provided by floating electrodes in theright ventricle and atrium in the presence of heart activity. Such heartactivity such as exercise can change the character of the signals actingupon the electrodes by displacement of the heart tissue in a way thatchanges the vector relationship of the signals and floating electrodes,as set forth in FIG. 3. But, this invention also prevents thedeterioration of sensed signals at a particular electrode set that couldcause erratic pacing from causing disfunction by switching to adifferent mode of operation of the pacemaker such as one responding todifferent heart activity measuring means. For example if sensor 76 findsarrythmia present in an atrial signal, it can substitute the signal 70produced from some other sensing means or control pulse sequence.

Thus the method of operation provided by this invention comprisesgenerating pacing pulses for stimulation of the heart via an electrodeimplanted in the heart, varying the pacing pulses in response toexercise activity of the patient in a normal P-wave triggered mode ofoperation and switching over the pacing mode in response to sensedabnormal P-wave conditions under control of a sensor. This can beachieved by means of selection of a proper subset of electrodes disposedin the heart for control purposes in accordance with a preferredembodiment of the invention.

In particular VDD pacing by means of floating electrode means in theatrium for detection of the atrial signal is a preferred manner ofnormal pacer operation, and in this mode of operation the vector signalsare processed to produce a more reliable sensing of real rather thanperceived atrial conditions in accordance with this invention. However asecond order of improvement under emergency conditions is afforded bythis invention, for example by sensing very high atrial rates indicatingfalse atrial tachycardia to compare with a different heart conditionsensor to correctly choose another mode or sensor that can bettercontrol the pacing rate.

Having therefore advanced the state of the art those features of noveltydescriptive of the nature and spirit of the invention are defined withparticularity in the following claims.

We claim:
 1. A rate varying pacemaker pacing control system comprising in combination, a single pacing probe having electrode means for stimulation of the ventricle of a patient and measuring electrode means for detecting signals in the atrium varying in response to the patient's activity, means for deriving a first P-wave control signal indicative of intrinsic atrial activity at the measuring electrode means, means for deriving a second rate control signal different from the atrial P-wave reflecting activity level of the patient, means for comparing the first and second rate control signals and selecting a preferred one of the signals for controlling the pacing rate, and pulse rate control means for varying the pacemaker rate in response to the preferred signal from said means for comparing the first and the second rate control signals, wherein the means for detecting signals provides signal components representative of the patient's breathing, and the means for deriving the second rate control signal includes frequency filtering means for accenting periodic amplitude variations of the intracardiac electrical signal representative of the patient's breathing.
 2. A rate varying pacemaker pacing control system comprising in combination, a single pacing probe having electrode means for stimulation of the ventricle of a patient and measuring electrode means for detecting signals in the atrium varying in response to the patient's activity, means for deriving a first P-wave rate control signal indicative of intrinsic atrial activity at the measuring electrode means, means for deriving a second rate control signal different from the atrial P-wave reflecting activity level of the patient, means for comparing the first and the second rate control signals and selecting a preferred one of the signals for controlling the pacing rate and pulse rate control means for varying the pacemaker rate, in response to the preferred signal from said means for comparing the first and the second rate control signals, wherein said means for deriving a second rate control signal further includes an additional set of two measuring electrodes respectively located on said probe for positioning in the right ventricle and in the right atrium for deriving the second rate control signal.
 3. A rate varying pacemaker pacing control system comprising in combination, a single pacing probe having electrode means for stimulation of the ventricle of a patient and measuring electrode means for detecting signals in the atrium varying in response to the patient's activity, means for deriving a first P-wave rate control signal indicative of intrinsic atrial activity at the measuring electrode means, means for deriving a second rate control signal different from the atrial P-wave reflecting activity level of the patient, means for comparing the first and the second rate control signals and selecting a preferred one of the signals for controlling the pacing rate, and pulse rate control means for varying the pacemaker rate in response to the preferred signal from said means for comparing the first and the second rate control signals, wherein the means for deriving the second rate control signal comprises means for detecting variations of electrical heart activity, and the means for deriving the second rate control signal further comprises electrodes on said probe for detecting electrical heart activity signals and a low frequency filter for processing the heart activity signals to derive a pulmonary signal from the electrical heart activity.
 4. A rate varying pacemaker pacing control system comprising in combination, a single pacing probe having electrode means for stimulation of the ventricle of a patient and measuring electrode means for detecting signals in the atrium varying in response to the patient's activity, means for deriving a first P-wave rate control signal indicative of intrinsic atrial activity at the measuring electrode means, means for deriving a second rate control signal different from the atrial P-wave reflecting activity level of the patient, and means for comparing the first and the second rate control signals and selecting a preferred one of the signals for controlling the pacing rate, and pulse rate control means for varying the pacemaker rate in response to the preferred signal from said means for comparing the first and second rate control signals, wherein said means for deriving a second rate control signal further comprises two second rate control signal sources, and logical selection means for deriving said preferred control signal from said two second rate control signal sources when said P-wave signal becomes marginal, and wherein said means for deriving a second rate control signal further comprises sensing means for location outside the heart.
 5. The pacemaker control system of claim 4 wherein said sensing means for location outside the heart further comprises means for providing signals responsive to a signal representing body activity of a patient excluding electrical heart activity and consisting of characteristics of the venous blood.
 6. A rate varying pacemaker pacing control system comprising in combination, a single pacing probe having electrode means for stimulation of the ventricle of a patient and measuring electrode means for detecting signals in the atrium varying in response to the patient's activity, means for deriving a first P-wave rate control signal indicative of intrinsic atrial activity at the measuring electrode means, means for deriving a second rate control signal different from the atrial P-wave reflecting activity level of the patient, means for comparing the first and the second rate control signals and selecting a preferred one of the signals for controlling the pacing rate and pulse rate control means for varying the pacemaker rate in response to the preferred signal from said means for comparing the first and the second rate control signals, wherein said means for deriving the second rate control signal comprises a detector providing signals from body activity of a patient other than the electrical heart signal.
 7. A rate varying pacemaker pacing control system comprising in combination, a single pacing probe having electrode means for stimulation of the ventricle of a patient and measuring electrode means for detecting signals in the atrium varying in response to the patient's activity, means for deriving a first P-wave rate control signal indicative of intrinsic atrial activity at a measuring electrode means, means for deriving a second rate control signal different from the atrial P-wave reflecting activity level of the patient, means for comparing the first and the second rate control signals and selecting a preferred one of the signals for controlling the pacing rate and pulse rate control means for varying the pacemaker rate in response to the preferred signal from said means for comparing the first and the second rate control signals, wherein said means for deriving a second rate control signal further comprises two second rate control signal sources, and logical selection means for deriving said preferred control signal from said two second rate control signal sources when said P-wave signal becomes marginal, and wherein the means for comparing and selecting signals is operable to select a preferred one of the three rate control signals.
 8. The method of pacing the heart with a pacemaker providing pacing pulses at a single pacing electrode in the ventricle comprising the steps of providing at least two alternative pacing control signals sensed as respective functions of physiological activity of a patient, evaluating the alternative signals to determine a preferred one for pacing rate control and controlling the pacing pulse rate as a function of the preferred one of the alternative signals in the pacemaker, and providing only one of said alternative signals from intracardiac electrical activity of the heart. 